Large aperture phased array antennas are used in various applications, including on commercial and military satellites. Nominal performance of large aperture phased array antennas requires either precise co-planar surfaces of the radiating elements or highly accurate knowledge of non-planar conditions so that appropriate electronic compensation can be applied. For example, nominal operation of a phased array antenna operating at a frequency f of 10 GHz with a wavelength λ of 3 cm depends on maintaining co-planar conditions of about 1/25λ=0.12 cm or 1.2 mm. With large aperture antennas (e.g., apertures>15 meters) a number of factors limit the ability of the structure to maintain such precise geometry, particularly in spacecraft applications.
Construction and testing of a large structure with mechanisms that can accurately deploy within 1/100 of 1% over the entire aperture is problematic. The effects of thermal expansion and contraction cause distortions in the structure and are a primary concern in design as well. Also, residual movement and deflection of the antenna structure during articulation is difficult to eliminate and add to the problem of maintaining surface accuracy as well as adding weight to the structure. Thus, as the aperture of the antenna increases, the need to incorporate electronic compensation techniques to acquire useful data from the antenna increases proportionally. The ability to effectively apply electronic compensation depends, largely, upon accurate knowledge of the surface of the antenna.
One manner of obtaining the required surface knowledge is to measure the surface, or several key points on the surface. Some measuring devices rely on angular measurements to determine the location of discrete points. However, small changes in the position of the points (e.g., changes on the order of 1.2 mm) are difficult to measure from long distances. For example, to detect a 1.2 mm change in position using a device 150 meters away from the point being measured requires the ability to measure an angular change of 1.2/150000 radians or 1.6 arc-seconds, which is a very small angle. Vibrometers and electronic autocollimators are good position sensors, but their use is typically limited to relatively short distances and small scan angles.